The number and shape of lattice solvent molecules controls spin-crossover in an isomorphous series of crystalline solvate salts

Abstract

Crystals of [FeL₂][BF₄]₂·nMeCN (L = N-(2,6-di{pyrazol-1-yl}pyrid-4-yl)acetamide; n = 1 or 2) and [FeL₂][ClO₄]₂·MeCN are isomorphous. When n = 1 the compounds exhibit an abrupt, hysteretic spin-transition below 200 K, but when n = 2 the material remains high-spin on cooling. [FeL₂]X₂·EtCN (X⁻ = BF₄⁻ or ClO₄⁻) are isomorphous with the MeCN solvates and undergo their spin-transition at almost the same temperature. However this now occurs in two-steps via a re-entrant mixed-spin intermediate phase, which correlates with crystallographic ordering of the bent propionitrile molecule.